1. Field of Invention
The present invention relates to alloys of beryllium and aluminum. More particularly, the invention describes a method for making alloys of aluminum containing beryllium and forming them into useful structural products.
2. Brief Description of the Prior Art
Alloys of aluminum and beryllium are known in the art. For example, Cooper U.S. Pat. No. 1,254,987 describes the addition of aluminum to beryllium for improving machinability. Fenn U.S. Pat. No. 3,337,334 discloses and claims the Lockalloy commercial product (developed by Lockheed and Berylco in the 1960's) which comprises aluminum base metal and 62 weight percent beryllium.
Lockalloy was produced in sheet form and incorporated into the ventral fin of the YF12 experimental aircraft (Duba, YF-12 Lockalloy Ventral Fin Program, Final Report, NASA CR-144971, 1976). Following the introduction of Lockalloy, extensive data was obtained on rolled alloys made from pre-alloyed aluminum having 62 weight percent beryllium. See, for example, London, Alloys and Composites, Beryllium Science and Technology, Volume 2, Plenum Press, New York (1979).
Second and third order elemental additions to aluminum-beryllium alloys are reported in the literature. They include additions of magnesium, silicon, nickel or silver for making ternary and quaternary alloys of aluminum and beryllium as described in McCarthy U.S. Pat. No. 3,664,889. These alloys are made from rapidly solidified alloy powder, consolidated and worked by conventional means. Russian work on ternary and higher order aluminum-beryllium alloys is variously described in Molchanova, Phase Equilibria in the Al--Be--Ni System at 600 Deg. C., Vest. Mosk. Univ. Khim., Vol. 27(3), pages 266-271 (1986); Komarov, Increasing the Strength of Welded Joints in an Al--Be--Mg Alloy by Heat Treatment, Weld. Prod., Vol. 26(1), pages 32-33 (1979); Kolachev, Constructional Alloys of Aluminum Beryllium and Magnesium, Metalloved. Term. Obrab. Metal. Vol. 13, pages 196-249 (1980); Nagorskaya, Crystallization in Al--Be--Mg--Zn Quaternary system Alloys, Metalloved. Term. Obrab. Metal., Vol. 9, pages 72-74 (1973).
Minor amounts of beryllium are typically added to aluminum-rich alloys to prevent oxidation of the aluminum and other alloy components during processing steps like melting and pouring. As a primary example, Brush Wellman Inc., Elmore, Ohio produces and distributes aluminum-rich master alloys containing 10 percent or less beryllium for further processing by bulk producers. The residual beryllium level in downstream aluminum product is preferably less than 0.01 percent.
The most current aluminum-beryllium phase diagram shows a simple eutectic with essentially no terminal, solid solubility at either end. This Al--Be phase diagram, adopted from Murray, The Aluminum-Beryllium System, Phase Diagrams of Binary Beryllium Alloys, ASM International Monographs on Alloy Phase Diagrams, page 9 (1987), is reproduced as FIG. 1 in this specification.
Brush Wellman has conducted extensive research on aluminum alloys containing from about 10 to about 75 weight percent beryllium. See Hashiguchi, Aluminum Beryllium Alloys for Aerospace Application, European Space Agency Structural Materials Conference, Amsterdam (March 1992). The research showed that an aluminum alloy of about 62 weight percent beryllium is about 70 volume percent beryllium, and an alloy of 50 weight percent beryllium is about 59 volume percent beryllium. It was also discovered that the density and elastic modulus of alloy compositions in this system follow the Rule of Mixtures, i.e., interpolation of alloy properties is generally possible between the respective properties of pure beryllium and pure aluminum.
Results from studies at Brush Wellman's Elmore facilities have also shown that large cast ingots and fine pre-alloyed atomized powder particles can be produced with microstructures showing a metal composite including beryllium in an aluminum matrix. Presently, Brush Wellman markets these alloys as extrusions and stamped sheet products under the trademark AlBeMet.TM..
All presently known processes for making aluminum based alloys containing beryllium require a complete melt down of the aluminum and beryllium starting materials. Aluminum and beryllium metal charges are liquified in a chamber lined with a refractory material, under vacuum at a temperature well above 1280.degree. C., the melting point of beryllium. This melt is usually cast into an ingot or atomized with an inert gas into a pre-alloy powder. Because these high temperature metallurgical processes are relatively expensive, they demonstrate a need for lower temperature methods which require less machining to reduce scrapped chip losses.
Brush Wellman has processed AlBeMet.TM. into useful component parts by two alternative routes. Both processes require vacuum melting of aluminum and beryllium starting materials in a ceramic-lined, refractory crucible at temperatures typically in the range between about 1350.degree. to about 1450.degree. C. In the first alternative, the liquified aluminum-beryllium melt is poured through a refractory nozzle to produce a stream which is intercepted by high velocity jets of an inert gas. The jets of gas break the liquid stream into tiny grains which solidify into a pre-alloy powder. Individual grains that comprise the powder pre-alloy have very fine dendritic micro-structure consisting of a beryllium phase within an aluminum alloy matrix. The pre-alloy powder is then consolidated by cold isostatic pressing, hot isostatic pressing or extrusion to produce a gross shape which can then be machined into a useful article.
The second alternative for processing AlBeMet.TM. into component parts is a conventional ingot casting operation in which molten aluminum-beryllium is poured into a graphite mold cavity and cooled to a solid ingot up to six inches in diameter. The microstructure of this casting is a relatively coarse, dendritic beryllium phase within an aluminum alloy matrix. The casting surface and hot-top are removed and scrapped and the ingot is further processed by rolling, extrusion or machining into the final article shape.
These alternatives are relatively expensive and cheaper net shaping processes are preferable. Conventional semi-solid processing or thixo-forming of metals takes advantage of low apparent viscosities obtained through continuous and vigorous stirring of heat-liquified metals during cooling. These techniques are generally described by Brown, Net-Shape Forming Via Semi-Solid Processing, Advanced Materials & Processes, pages 327-338 (January 1993). Various terms are presently used to describe semi-solid processing of metals to form useful articles of commerce, including rheo-casting, slurry-casting, thixo-forging and semi-solid forging. Each of these terms is associated with variations in the steps during semi-solid processing or in the types of equipment employed.
Semi-solid processing is initiated by heating a metal or metals above their liquidus temperatures to form molten metal or alloy. Various methods known in the art are used to introduce shear forces to the liquified metals during slow cooling to form, in situ, equiaxed particles dispersed in the melt. Under these conditions, the metals are said to be in a "thixotropic" or semi-solid slurry state. Thixotropic slurries are characterized by non-dendritic microstructure and can be handled with relative ease by mass production equipment allowing process automation and precision controls while increasing productivity of cast materials. See Kenney, Semisolid Metal Casting and Forging, Metals Handbook, 9th Ed., Vol. 15, pages 327-338 (1988).
The non-dendritic microstructure of semi-solid metal slurries is described in Flemings U.S. Pat. No. 3,902,544 which represents the state of this art. The described method concentrates on vigorous convection during slow cooling to achieve the equiaxed particle dispersion leading to non-dendritic microstructure. See also, Flemings, Behavior of Metal Alloys in the Semisolid State, Metallurgical Transactions, Vol. 22A, pages 957-981 (1991).
Published literature prior to this disclosure has focused on the magnitude of force required to deform and fragment dendritic growth structures using high temperature shearing. It was discovered that semi-solid alloys displayed viscosities that rose to several hundreds, even thousands, of poise depending on shear rates (Kenney, Semisolid Metal Casting and Forging, Metals Handbook, 9th Ed., Vol. 15, page 327 (1988)), and that the viscosity of a semi-solid slurry, measured during continuous cooling, was a strong function of applied shear forces--with measured viscosities decreasing as shear rate increased. Flemings, Behavior of Metal Alloys in the Semi-Solid State, ASM News, pages 4-5 (September 1991).
Subsequent commercial processes focused on developing different ways to agitate liquified metals to achieve the roughly spherical or fine-grained microstructure in semi-solid slurry. Two general approaches have been developed--(1) rheo-casting, in which a slurry is produced in a separate mixer and delivered to a mold and (2) semi-solid forging, in which a billet is cast in a mold equipped with a mixer which creates the spherical microstructure directly within the mold.
Winter U.S. Pat. No. 4,229,210 discloses a method for inducing turbulent motion when cooling metals with electro-dynamic forces in a separate mixer; while Winter U.S. Pat. Nos. 4,434,837 and 4,457,355 disclose a mold equipped with a magneto-hydrodynamic stirrer.
Various methods for agitating or stirring have been developed to introduce shear forces in the cooling metals to form semi-solid slurry. For example, Young U.S. Pat. No. 4,482,012, Dantzig U.S. Pat. No. 4,607,682 and Ashok U.S. Pat. No. 4,642,146 describe means for electromagnetic agitation to produce the necessary shear forces within liquified metals. Mechanical stirring to produce shear rates are also described in Kenney U.S. Pat. No. 4,771,818, Gabathuler U.S. Pat. No. 5,186,236 and Collot U.S. Pat. No. 4,510,987.
Application of currently known semi-solid processing technology to alloys of aluminum containing beryllium is difficult because the dendritic structures present in pre-alloyed materials require extremely high temperature thixotropic processing under negative vacuum pressure. These high temperatures must exceed the melting point of beryllium (1280.degree. C.).
The present specification describes solutions to the stated problems for making alloys of aluminum containing beryllium and further discloses an improvement in semi-solid processing for metal alloys.